1. Field of the Invention
This invention relates to methods for forming one or more screen pack assemblies for use in filtering operations and, more particularly, to methods for forming one or more screen packs which may be employed in filtering out foreign matter during the extrusion or other processing of plastic materials.
2. Description of the Prior Art
The use of screen pack assemblies as filtering agencies, e.g., for filtering out foreign matter from extruded plastic materials, is known. Such screen packs are composed of a number of typically circular, screen sections, e.g., three, which are suitably oriented with each other, and which together form a single, united structure.
Screen packs have previously been formed by first punching out a number of circular sections from a sheet of screening; then grouping the screen sections in sets, e.g., of three screens each; orienting the screen sections in each set with one another; and thereafter uniting the screen sections in each set into a single screen pack assembly, e.g., by spot welding operations or by securing a retaining ring about the circumference of each set of screen sections. Such technique for forming screen packs is slowed down considerably by the need for numerous, successive, screen section grouping, orienting and uniting operations. Thus, the provision of a faster, more efficient technique for assembling a screen-pack type filtering agency, avoiding the repetitive, time-consuming, grouping-orienting-uniting sequence after formation of the screen sections, is considered quite desirable.
Agencies other than screen packs, which serve to filter out foreign matter from extruded, or otherwise formed, plastic material, are also known. Thus, sand and similar particulate materials have been employed for filtering purposes. The sand may initially be bound together by a plasticizing agent or a thermoplastic material, as taught by U.S. Pat. Nos. 2,607,954 and 3,630,384 to G. Schneider et al. and H. Toda, respectively. The use of sand as a filtration medium is considered disadvantageous, however, due to the fact that sand based filters provide the equivalent of extremely fine mesh screens, removing very minute particles from the plastic materials being filtered, but also tending to become rapidly clogged by such particles. Thus, the frequency of filter-changing operations need be greatly increased if sand is to be substituted for screening as a filtration medium.